INFRARED-VISIBLE SUM FREQUENCY GENERGATION STUDIES
OF WATER AT THE POLYMER/SAPPHIRE INTERFACE
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements of the Degree
Master of Science
Jing Zhou
August, 2013
INFRARED-VISIBLE SUM FREQUENCY GENERGATION STUDIES
OF WATER AT THE POLYMER/SAPPHIRE INTERFACE Jing Zhou
Thesis
Approved:
Accepted:
Advisor
Dr. Ali Dhinojwala
Dean of the College
Dr. Stephen Z. D. Cheng
Faculty Reader
Dr. Mesfin Tsige
Dean of the Graduate School
Dr. George R. Newkome
Department Chair
Dr. Coleen Pugh
Date
ii ABSTRACT
Water between two solid surfaces plays an important role in interfacial adhesion,
catalysis, corrosion, microelectronic industry and biomaterials. (1, 2) However,
research on molecular-level structure of confined water between two solid surfaces
and the disruption of interactions between these two surfaces caused by water are
lacking. Infrared-visible sum frequency generation spectroscopy (SFG) is used to
directly probe confined water between polyurethane (PU) and the sapphire substrate
after exposing the polyurethane films to liquid water and water vapor. In liquid water
condition, the observation of SFG peaks associated with H2O bands (3000-3400 cm-1)
and D2O bands (2300-2600 cm-1) indicate water molecules ingress to the buried
interface and exist in the form of hydrogen-bonded water network. The water layer
disrupts interactions between polyurethane and hydroxyl groups on the substrate.
When PU films were exposed to water vapor, the SFG signal corresponding to PU
hydrocarbon groups significantly increase, while the SFG signal of sapphire hydroxyl
groups decrease, which indicates water molecules reach the interface. However, no
hydrogen-bonded water network was observed, instead, H2O peak at 3555 cm-1 and
D2O peaks (2600-2700 cm-1) show up which can be assigned to low-coordination
water. An alternate assignment could be the hydroxyl groups hydrogen bonded with
carboxyl groups of PU. Water molecules cannot form a uniform monolayer at the
interface and as a consequence cannot completely disrupt the PU-sapphire bonds.
These results provide two distinct states of water at the polymer-solid interface, which
iii could influence the interfacial bonding state differently and have important
implications in understanding interfacial adhesion, coatings and corrosion.
iv ACKNOWLEDGEMENTS
I would like to thank my advisor, Dr. Ali Dhinojwala, for his encouragement,
support and guidance to my research. He gave me the chance to fulfill my dream and
enjoy the scientific exploration. I appreciate all the group members for discussing and
sharing their knowledge with me. We had a great time working together in the past
three years. In particular, I want to thank the whole SFG group for assisting me in
SFG experiments.
I am very grateful to my parents for their support. With everything they made
available to me, I could pursue my dream without any burden. I also appreciate all of
my friends for sharing happiness and pain with me. They gave me good memories in
Akron.
Finally, I would like to express my immense gratitude to my boyfriend Fei Lin
for his love and care. He never stopped supporting me even in the most difficult time.
This achievement would not have been possible without him.
v TABLE OF CONTENTS
Page
LIST OF TABLES......................................................................................................viii
LIST OF FIGURES ......................................................................................................ix
CHAPTER
I.
INTRODUCTION.............................................................................................. 1
II.
BACKGROUND ................................................................................................ 6
III.
2.1
Water Confined Between Two Solid Surfaces ........................................... 6
2.2
Infrared-Visible Sum Frequency Generation Spectroscopy ..................... 13
2.2.1
Basic Principles ........................................................................... 14
2.2.2
SFG Study of Polymers............................................................... 23
2.2.3
SFG Study of Water .................................................................... 24
EXPERIMENTAL............................................................................................ 35
3.1
Sample Preparation .................................................................................. 35
3.2
Ellipsometry Measurements..................................................................... 36
3.3
Quartz Crystal Microbalance (QCM) Measurements .............................. 36
3.4
Sum Frequency Generation Spectroscopy Apparatus and Measurement 37
3.5
Total Internal Reflection (TIR) Geometry Combined with SFG
Measurement............................................................................................ 40
vi 3.6
IV.
Sample Cell and the Humidity Controlling Set-Up ....................................... 41
RESULTS AND DISCUSSION ....................................................................... 43
4.1
Calculating Contributions of SFG Signal from Two Interfaces as a
Function of the Film Thickness ................................................................ 43
4.2
The Thickness of Polymer Films by Ellipsometry ................................... 45
4.3
Water Uptake of Polymer Films by QCM ................................................ 45
4.4
PU-Air Interface and PU-Sapphire Interface of Samples in the Ambient
Condition by SFG ..................................................................................... 47
4.5
PU-Water Interface and PU-Sapphire Interface after Exposure of Samples
to Liquid Water by SFG............................................................................ 51
4.6
Sapphire-Water Vapor (or Dry N2 Gas) Interfaces by SFG ..................... 55
4.7 PU-Sapphire Interface after Exposure of Samples to Water Vapor by SFG
.................................................................................................................................. 57
4.8
V.
The Effect of Hydrophobicity for Polymer Coating on Water Transport. 62
CONCLUSION.................................................................................................. 65
BIBLIOGRAPHY ....................................................................................................... 67
APPENDIX ................................................................................................................. 77
vii LIST OF TABLES
Table
Page
4.3-1. QCM experimental data and calculated data................................................... 47 4.4-1. SFG fitting parameters with PPP polarization deduced from Figure 4.4-1 for
the experiments of probing the PU-air interface and the PU-sapphire interface.
......................................................................................................................... 49 4.4-2. SFG fitting parameters with SSP polarization deduced from Figure 4.4-2 for
the experiments of probing the PU-air interface and the PU-sapphire interface.
......................................................................................................................... 49 4.5-1. SFG fitting parameters with PPP polarization deduced from Figure 4.5-1 for
the experiments of probing the PU-H2O interface and the PU-D2O interface. 51
4.5-2. SFG fitting parameters with PPP polarization deduced from Figure 4.5-2 for
the liquid H2O and D2O experiments of probing the PU-sapphire interface. .. 54 4.7-1. SFG fitting parameters with PPP polarization deduced from Figure 4.7-1 for
the water vapor experiment of probing the PU-sapphire interface at various
RH (%) of H2O vapor. ..................................................................................... 59 4.7-2. SFG fitting parameters with PPP polarization deduced from Figure 4.7-2 for
the water vapor experiment of probing the PU-sapphire interface at various
RH (%) of D2O vapor. ..................................................................................... 61 4.7-3. Relevant peak assignments for H2O, D2O and PU next to various interfaces. 61
viii LIST OF FIGURES
Figure Page
2.1-1. Force between two mica surfaces in mM KCl solutions as a function of
separation distance. Below 2 nm, the force is oscillating with a periodicity of
the diameter of water molecules (2.5 ). Above 2 nm, the force is repulsive.
Inset: theoretical prediction of force for this system based on a non-continuum
molecular theory. (29)........................................................................................ 7
2.1-2. Whole-animal shear (frictional) adhesion forces from tokay geckos tested on
four surfaces in dry or wet contact. Each gecko (n = 6) was tested three times
on each surface (glass, PMMA, OTS-SAM–coated glass, and PTFE), and the
highest of the three tests was used for data analysis. Surfaces were tested
either without water (dry) or fully submerged in water (wet). (30)................... 8
2.1-3. (a) The model for the impedance of a polymer coated metal and (b) theoretical
impedance spectra for a degraded polymer coating. (31).................................. 9
2.1-4. Solid points represent the moisture uptake into thin PnOMA films coated on
quartz crystals. corresponds to the adsorption on the blank crystal. The
dashed lines are linear fits to the data. (17) ..................................................... 10
2.1-5. The X-ray reflectivity profiles for polyacrylate layer on Al2O3 with different
curing conditions in vacuum (gray curve) and exposed to saturated H2O vapor
at 60
(black curve). The reflectivity curves are offset for clarity. (18)..... 11
2.1-6. The Neutron-scattering length density profile from the fitting of the Neutron
reflectivity profiles for samples (polyacrylate layer on Al2O3) in vacuum (solid
line) and exposed to saturated water vapor (dash line). (18) ........................... 12
2.1-7. A model illustrating the contact region formed between the PDMS lens and the
sapphire surface in the presence of confined water. A tentative physical model
of the molecular structure in these two types of contact area s is also shown in
the panels on the right. The relative sizes of these two regions cannot be
quantified from measurements. (32)................................................................ 13
ix 2.2-1. Schematic of the laser beams for SFG spectroscopy at an interface. (35) ....... 17
2.2-2. Definition of Euler angles relating lab axes to molecular axes........................ 21
2.2-3. The SFG spectra of the vapor/water interface, IR spectra of bulk water and
bulk ice, and molecular structure of hexagonal ice. Upper-left: The SFG
spectrum of the vapor/water interface shows three OH bands at 3200, 3400
and 3700 cm-1. Upper-right: side view of the bulk ice near the (0001) surface.
Bottom-left: The IR spectrum of bulk ice shows a dominant peak at 3150 cm-1.
Bottom-right: The IR spectrum of bulk water is dominated by a broad peak at
~3400 cm-1. (60) .............................................................................................. 25
2.2-4. The SFG spectra of the vapor/water interface taken with (a) SSP, (b) PPP, and
(c) SPS polarization combinations. (61) .......................................................... 27
2.2-5. The Im
spectrum of HOD at the vapor/water interface probed by phasesensitive sum frequency generation spectroscopy, and schematic of the first
two molecular layers at the vapor/water interface occupied by DDA, DAA and
DDAA water molecules. (70) .......................................................................... 28
2.2-6. The SFG spectra of water (D2O) film on mica as a function of the relative
humidity (RH) at room temperature (296K) with SSP polarization, and
scanning polarization force microscopy images, showing 2-dimensional
islands of water (bright patches) produced by a brief contact of the atomic
force microscope tip near the center of the image, that induces capillary
condensation around the contact point. At relative humidity values below
70%, the clusters show a contrast of 2.5
0.5 Å. The contrast decreases
down to the noise level at high humidity (80%). (71) ..................................... 30
2.2-7. Possible hydrogen-bonding configuration of water molecules on hydrophilic
silica surface: (a) protonated (SiOH) surface sites, low pH; (b) deprotonated
(SiO-) surface sites, high pH; (c) structure of water/silica interface at low pH.
Red and gray spheres represent O and H atoms of water molecules; large
graygreen, pink, and white spheres represent Si, O, and H atoms of SiOH
groups at silica surface. Dotted lines indicate hydrogen bonds. (72) .............. 31
2.2-8. The SFG spectra the water/fused silica interface as a function of pH.
Polarization combination is SSP. A spectrum of the ice/fused silica interface is
shown for comparison (filled squares). The spectra are offset vertically by 2
units for clarity. (73) ........................................................................................ 32
x 2.2-9. The SFG spectra of the water/hydrophobic interfaces: (a) the water /
octadecyltrichlorosilane (OTS) covered silica interface; (b) the water / vapor
interface; (c) the water/hexane interface. (80) ................................................. 33
3.1-1. The chemical structure of polyurethane that is used in our study. ................... 36
3.4-1. The schematic diagram of SFG apparatus........................................................ 39
3.5-1. The schematic of combining TIR geometry with SFG measurements. ........... 40
3.6-1. Diagram of the sample geometry for SFG measurements. Liquid water and
water vapor can be sealed well in the gap between the sapphire prism and the
stainless steel sample cell. ............................................................................... 41
3.6-2. Diagram of the humidity-controlling set-up. The water container is made of
glass and other parts are made by stainless steel. ............................................ 42
4.1-1. Predictions of interferences of the SFG signal for two interfaces as a function
of the thickness of polymer films by a three-layered structural model. Red
dashed line is the ratio of signals from PU-air interface over PU-sapphire
interface at incident angle 2 degrees. Blue dashed line is the ratio of signals
from PU-water interface over PU-sapphire interface at incident angle 2
degrees. Pink solid line is the ratio of signals from PU-sapphire interface over
PU-air interface at incident angle 42 degrees. Black solid line is the ratio of
signals from PU-sapphire interface over PU-air interface at incident angle 16
degrees. ............................................................................................................ 44
4.2-1. Diagram of the thickness of PU films as a function of the solution
concentrations varying from 0.3% to 1%. Each data point is the average result
of three different samples. ............................................................................... 45
4.3-1. The QCM spectra of the bare sensor and the sensor spin coated with the
polyurethane film (300nm by ellipsometry) at 3rd, 5th, 7th and 9th overtones. . 46
4.4-1. The SFG spectra in PPP polarization for the PU-air interface (a) and the PUsapphire interface (b). The solid lines are fit to a Lorentzian equation. .......... 48
4.4-2. The SFG spectra in SSP polarization for the PU-air interface (a) and the PUsapphire interface (b). The solid lines are fit to a Lorentzian equation. .......... 50
xi 4.5-1. The SFG spectra in PPP polarization for the PU-water interfaces: (a) the PUH2O interface and (b) the PU-D2O interface. The solid lines are fit to a
Lorentzian equation. ........................................................................................ 52
4.5-2. The SFG spectra in PPP polarization for the PU-sapphire interface upon
exposure to liquid water. Both H2O (a) and D2O (b) reaches the PU-sapphire
interface. The solid line in (a) is a guide to the eye. The solid line in (b) is fit
to a Lorentzian equation. ................................................................................. 53
4.5-3. The SFG spectrum in PPP polarization for the D2O/sapphire interface. The
solid line is used to guide eyes......................................................................... 55
4.6-1. The SFG spectra of the Sapphire-water vapor (or dry N2 gas) interface in PPP
polarization after the humidity-controlling set-up was integrated to the sample
cell. The solid lines are used to guide eyes...................................................... 57
4.7-1. The SFG spectra of the PU-sapphire interface that are collected at various RH
of H2O vapor in PPP polarization. These spectra are collected at 0%, 23%,
48% and 82% RH, respectively. The solid lines are fitted to a Lorentzian
equation............................................................................................................ 58
4.7-2. The SFG spectra of the PU-sapphire interface that were collected at various
RH of D2O vapor. These spectra are collected at 0%, 23%, 53% and 80% RH,
respectively. The solid lines are fitted by Lorentzian equation. ...................... 60
4.8-1. The contact angle images of the water droplet on (a) the polyurethane-coated
sample and (b) the fluorinated layer-coated sample. ....................................... 62
4.8-2. The SFG spectra of the PU-sapphire interface (of plasma treated samples) that
were collected at various RH of D2O vapor. These spectra are collected at 0%,
23%, 49% and 69% RH, respectively. The solid lines are guides to the eyes. 63
5-1.
A model illustrating the PU-sapphire interface in the presence of water
molecules under liquid water and water vapor conditions, respectively. ........ 65
A1.
The IR spectrum of PU in the ambitious condition. ......................................... 77
xii CHAPTER I
INTRODUCTION
The incorporation of water between two solid surfaces plays an important role
in many areas such as interfacial adhesion, corrosion, catalysis and lubrication. (1, 37) Interfaces that consist of various organic polymers combining with inorganic
substrates are of technological consequence in understanding systems like protective
coatings, adhesive joints, and polymer/inorganic composites. (8, 9) When such
systems are exposed to liquid water or exposed to high humidity, water can ingress
into the polymer-substrate interfaces by diffusing through intact polymer films or
along the interfaces. (10, 11) As a result, water replaces the initial bonds between
polymer and substrate and even causes delamination and mechanical failure, which is
detrimental to the durability of polymer/substrate systems. (12)
Various non-destructive measurements have been developed recently for
probing water transport underneath polymer films such as electrochemical impedance
spectroscopy (EIS), Fourier transform infrared multiple internal reflection (FTIRMIR) spectroscopy, quartz crystal microbalance (QCM), and X-ray and neutron
reflectometry (XR and NR). Wormwell and Brasher have utilized EIS to measure
electrical properties of organic coatings after exposure to aqueous environments since
early 1950s. (13) The analysis of EIS data is based on building proper electric-circuit
models and thus depends on the validity of models. FTIR-MIR spectroscopy was
developed by Nguyen et al. that enabled in situ and quantitative detection of water in
vicinity of the substrate. (14, 15) In this technique, the probing depth is 1.75
1 and
thus detectable water molecules include water layer at the polymer-substrate interface
and water absorbed in the polymer film within the probing depth. Even though FTIRMIR is not interface specific, it provides information on water transport pathways and
water-susceptibility of the polymer-substrate interface helping to understand the
mechanisms of water-induced adhesion loss of polymer-coated systems.
QCM, XR and NR were also used to study the absorption of water underneath
polymer films. Because of better resolution of these techniques than EIS and IR, ultra
thin films from tens to hundreds nanometers were used in these studies. QCM were
widely used to measure water uptake in a variety of polymer-substrate systems as it’s
sensitive to even several nanometers thick water absorption. (16, 17) More recently,
Vogt et al. (18, 19) applied NR and XR to study water transport kinetics in
multilayered systems upon exposure to water vapor. The reflectivity data described
the moisture permeation process and swelling of individual polymeric layers in detail.
NR measurements with D2O provided information of the water distribution within
each layer and water concentration accumulated at the interface. All these techniques
used to monitor water transport in polymer-coated systems are of practical importance
for predicting barrier properties of polymer coatings and understanding the impact of
water on interfacial properties such as adhesion and electrochemistry.
Previous studies showed that water accumulation at the polymer-substrate
interface depends on substrate surface chemistry which influences the interaction with
polymer coatings and water. Nguyen et al. pointed out that plasma treated substrate
with enhanced interactions with PMMA lead to a decreased amount of water at the
interface. (15) O’Brien et al. found that hydrocarbon treatments which made the
substrate more hydrophobic and hence weaker interactions with water suppressed the
water accumulation at the interface. (19) An investigation was conducted by Karul et
2 al (17) to clarify the effect of polymer chain mobility on water accumulation content.
They found that rubbery polymers are able to suppress the accumulation of moisture
at interface with the explanation that the low-Tg polymers (PnBMA and PnOMA)
promote relatively unrestricted molecular movement towards a thermodynamically
stable conformation such that the hydrophobic alkyl chains are closer to the interface.
Although a variety of studies for quantifying the interfacial water have been
conducted, the structure of this complex water interface is not understood.
Understanding molecular structure of the polymer-substrate interface in the presence
of water can help to clarify the mechanism of water-induced interfacial properties. To
address this question, we have studied the confined water between the polymer filmsapphire (Al2O3) interfaces using infrared-visible sum frequency generation
spectroscopy (SFG) in total internal reflection (TIR) geometry. (20) SFG is a
nonlinear optical spectroscopic technique that is highly surface-specific because
second order nonlinear optical processes are forbidden in centrosymmetric media
under electric-dipole approximation. (21) This makes it possible to probe various
surfaces and interfaces where inversion symmetry is broken. (22-24) Briefly, SFG
technique involves two incident beams, one visible beam
and a tunable IR beam
, overlapping temporally and spatially on the sample and generating a sum
frequency signal ( ω 1 + ω 2 ) at the interface. The SFG signals are enhanced when the
scanning IR frequency overlaps with the molecular vibrational modes of molecules at
the interface that are infrared and Raman active. The intensities and positions of peaks
can provide chemical and orientation information of molecules. (25) Combining with
total internal reflection geometry, SFG spectroscopy permits us to study the presence
of water between the polymer-sapphire interface. By combining SFG with TIR
geometry, it enables us to selectively probe the individual interface in a multilayered
3 system and the SFG output is enhanced 10-100 times when the incidence angles are
near the critical angle for TIR. (26, 27) Taking advantage of this approach, we studied
the structure of the polyurethane (PU)-sapphire interface upon exposure to liquid
water and water vapor with controllable relative humidity (RH). This technique can
also be applied to a variety of polymer-substrate systems for monitoring the ingress of
water to the coating-protected interface mimicking the natural environments such as
rain and humid air.
In this dissertation, we chose polyurethane as a model polymer for our study
because polyurethane is widely used in the automotive refinish and large vehicle
coating areas, and in metal, wood, plastic, glass, and textile coating. (28) We have
addressed three main questions in this research. First, can water ingress to the
polymer-sapphire interface? Second, what is the structure of water in this confined
geometry? Third, what are the effects of water at the interface?
Some basic concepts of water between two solid substrates are discussed in
Chapter 2. It also provides a brief insight on our current understanding of this subject.
In Section 2.2, we will provide an introduction to the principles of non-linear optics
and the theoretical framework for SFG. At the end of Chapter 2, typical studies of
water at various interfaces are presented to highlight the major advantages of this
spectroscopic technique.
Chapter 3 discusses the sample preparation and experimental techniques in
this study. It also provides details of the development of the sample cell and the
humidity-controllable set-up.
Chapter 4 provides the results and discussion. To understand the ingress of
water to the polymer-sapphire interface, we exposed samples in both liquid water and
water vapor. H2O and D2O were used in the study to confirm our finding. At the end
4 of Chapter 4, the effect of hydrophobicity of the organic layer on preventing water
penetration was presented.
Finally, in Chapter 5 we conclude with a summary of the major results of this
research.
5 CHAPTER II
BACKGROUND
2.1
Water Confined Between Two Solid Surfaces
In nature, we can see confined water in many areas such as gecko pads- wet
solid substrate interfaces and cellular membrane interfaces. In industry, confined
water layer involving in areas like coating-metal interfaces and tire-road interfaces is
everywhere around us. Confined water in those systems plays very important roles in
adhesion, corrosion, lubrication and friction.
It’s not easy to measure the properties of water confined between two solid
substances because of this indirect experimental geometry. Israelachvili and
coworkers used surface force apparatus to study the force between two smooth solid
surfaces in the presence of water. At long range, a repulsion force exists between the
two solids and it increases exponentially with decreasing separation distance, which is
well described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.
Measurements at short range reveal oscillations in the force with a period ~0.25nm
which corresponds to the size of a water molecule. When the separation is
approaching 0, the force is expected to be attractive. (29) Figure 2.1-1 shows the
measured force as a function of separation distance and it suggests that forces
between surfaces are related to water structure, especially for the first layer of water
in direct contact with surfaces.
6 Figure 2.1-1. Force between two mica surfaces in mM KCl solutions as a function of
separation distance. Below 2 nm, the force is oscillating with a periodicity of the
diameter of water molecules (2.5 ). Above 2 nm, the force is repulsive. Inset:
theoretical prediction of force for this system based on a non-continuum molecular
theory. (29)
Mimicking gecko adhesion on wet natural surfaces, Alyssa and coworkers
tested the adhesion of geckos on submerged substrates with various surface
wettabilities. (30) They found that (in Figure 2.1-2) geckos significantly lose their
adhesion on a wet hydrophilic surface compared with a dry hydrophilic surface. They
didn’t see adhesion difference between dry and wet contact of an intermediated
hydrophobic surface. However, geckos clung significantly better to a wet
hydrophobic substrate than the dry substrate. Their experimental and calculated
results suggested that gecko adhesion is highly dependent on surface wettability and
7 the presence of water or air between the toe pad and the contact surface. This study
provides us direct evidence of water’s impacts on varying interfacial properties in
natural environment, and highlights the importance of studying water confined
between two surfaces.
Figure 2.1-2. Whole-animal shear (frictional) adhesion forces from tokay geckos
tested on four surfaces in dry or wet contact. Each gecko (n = 6) was tested three
times on each surface (glass, PMMA, OTS-SAM–coated glass, and PTFE), and the
highest of the three tests was used for data analysis. Surfaces were tested either
without water (dry) or fully submerged in water (wet). (30)
As we all know, water penetrating protective coatings and reaching the
coating-metal interface will potentially initiate corrosion. EIS was widely used to
evaluate corrosion protection of metals and alloys by polymer coatings. Those studies
include evaluation water uptake of coatings, degradation of coatings with exposure
time, disbanding of coatings, determination of the active area at which corrosion
occurs, and estimation of corrosion rates at the metal /coating interface. (31) As
shown in Figure 2.1-3 (a), the analysis of impedance data (in Figure 2.1-3 (b)) of
8 polymer-coated metal exposed to corrosive media is based on the simple model where
,
,
,
and
correspond to the uncompensated resistance between
reference electrode and test electrode, the capacitance of the polymer coating, the
“pore resistance” resulting from the formation of ionic conducting paths across the
coating, and the polarization resistance of the area at the metal-coating interface at
which corrosion occurs and the corresponding capacitance. However, the reliability of
analyzed data depends on the validity of the model that was used for analyzing.
Figure 2.1-3. (a) The model for the impedance of a polymer coated metal and (b)
theoretical impedance spectra for a degraded polymer coating. (31)
9 Recently, the detection and quantification of water at the polymer-solid
interface can be achieved by QCM, NR and XR techniques. The mechanism of
quantifying water by QCM is based on relating resonance frequency of piezoelectric
sensors to the change in mass experienced by sensors because of molecular absorption.
However, the mass change measured by QCM includes mass changes caused by
absorption in the bulk film and molecules at the polymer-solid interface. The absolute
quantification for the interface can be achieved by extrapolating of the uptake to zero
thickness as shown in Figure 2.1-4. (17)
Figure 2.1-4. Solid points represent the moisture uptake into thin PnOMA films
coated on quartz crystals. corresponds to the adsorption on the blank crystal. The
dashed lines are linear fits to the data. (17)
The reflectivity-based metrology provides details about the moisture
permeation process through multilayer stacks. Polymer films swelling upon exposure
to water can be detected by XR as shown in Figure 2.1-5. (18) Neutron reflectivity
measurements with deuterated water are used to provide information of the water
distribution within each layer. The amount of D2O at the buried interface can be
10 quantified by the change in the scattering length density after moisture exposure. The
scattering length density profiles corresponding to the fit of the NR data are shown in
Figure 2.1-6. (18)
Figure 2.1-5. The X-ray reflectivity profiles for polyacrylate layer on Al2O3 with
different curing conditions in vacuum (gray curve) and exposed to saturated H2O
vapor at 60
(black curve). The reflectivity curves are offset for clarity. (18)
Understanding the structure of water helps clarify the mechanism how water
influences the contact interfaces. Kumar and coworkers deformed a elastomeric
PDMS lens against a flat solid surface to study confined water between them by
infrared-visible sum frequency generation spectroscopy. (32) They correlated
molecular structure of the contact region with friction of the system. They found that
even though the sliding of PDMS lens on the solid substrate in the presence of water
generated lower friction than dry sliding, the friction coefficients of wet sliding were
much higher than those expected for a contact spot with a uniform layer of water
(lubricated sliding). The origin of the higher friction coefficient was attributed to the
11 Figure 2.1-6. The Neutron-scattering length density profile from the fitting of the
Neutron reflectivity profiles for samples (polyacrylate layer on Al2O3) in vacuum
(solid line) and exposed to saturated water vapor (dash line). (18)
patchy contact spot with regions where the elastomer was in direct contact with the
solid surface in literature. (7) This hypothesis was proved in this study by observing
specific vibrational bands that illustrated molecular structures in the contact area (as
shown in Figure 2.1-7).
However, the direct measurements of the structure of water confined between
two solid surfaces are still lacking, especially for the water layer in atomic scale.
Taking advantage of SFG spectroscopy, we are able to investigate confined water
between two solid surfaces (polymer film and sapphire prism) in our study. Studying
water in this system is very important for mimicking coating-metal interfaces in the
presence of water. For example, coatings on aluminum are often exposed to humidity
and rain and the study of these coatings to these environmental conditions is
12 important in preventing corrosion. The early detection of water at interfaces will help
in designing polymers for corrosion protection.
Figure 2.1-7. A model illustrating the contact region formed between the PDMS lens
and the sapphire surface in the presence of confined water. A tentative physical model
of the molecular structure in these two types of contact area s is also shown in the
panels on the right. The relative sizes of these two regions cannot be quantified from
measurements. (32)
2.2
Infrared-Visible Sum Frequency Generation Spectroscopy
In recent years, surfaces and interfaces science attract intensive attentions
because of their important roles in many areas, such as interfacial adhesion, catalysis,
electrochemistry and corrosion. However, there are still limitations of available
13 surfaces and interfaces probe techniques. Many electron probe techniques required
ultrahigh vacuum (UHV) conditions e.g. Low-energy electron diffraction (LEED) and
X-ray photoelectron spectroscopy (XPS), which limited their applications to dry
sample surfaces. (33) Microscopic probes such as scanning tunneling microscopy
(STM) and atomic force microscopy (AFM) can be applied to study samples with
liquid on top, but suffer from molecular movements at liquid interfaces. (21) Some
other techniques that have been used to study surfaces and interfaces e.g. X-ray
spectroscopy and Neutron reflectivity are not highly surface-specific. On the other
hand, second-order nonlinear optical spectroscopy, including second harmonic
generation (SHG) and sum frequency generation (SFG), shows its advantages in
surfaces and interfaces studies. It’s highly surface-specific because nonlinear optical
processes
are
forbidden
in
centrosymmetric
media
under
electric-dipole
approximation and only permitted at surfaces and interfaces where inversion
symmetry is broken. (21)
2.2.1
Basic Principles
In 1961, the discovery of second-harmonic generation by Franken et al.
opened the world of nonlinear optics. The phenomena are called nonlinear optics
when a material responses to an applied optical field in a nonlinear manner. To
describe more precisely what optical nonlinearity is, we can consider how the dipole
moment per unit volume, or polarization
of a material depends upon the strength
of the applied optical field.
In the case of linear optics, the induced polarization shows linear relationship
with the electric field strength (34)
14 P(t) = χ (1) E(t)
(2.1)
where
can
is the linear susceptibility. In nonlinear optics, the polarization
be expressed by a series of orders of the electric field strength
.
(2.2)
The quantities
and
are the second-order and the third-order nonlinear
optical susceptibilities, respectively. The second-order nonlinear optical interactions
can only occur in noncentrosymmetric media. On the other hand, the third-order
nonlinear optical interactions can occur both in noncentrosymmetric and
centrosymmetric media.
A laser beam whose electric field strength is represented as
E(t) = Ee−iω t + c.c.
(2.3)
is incident on a medium with a nonzero second-order susceptibility
. The medium
can generate polarization as
P (2) (t) = 2 χ (2) EE * + ( χ (2) E 2 e−2ω t + c.c.)
where
is the charge of an electron,
(2.4)
is the frequency and
is the time. This is the
basic principle of SHG. Under proper experimental conditions, the incident beam with
15 frequency
can be effectively converted to radiation with second harmonic
frequency
.
For sum frequency generation, the incident laser beams consist of two distinct
frequencies. The electric fields of incidence can be represented as
E(t) =
1
(E1e−iω1t + E2 e−iω 2 t + c.c.)
2
(2.5)
The generated nonlinear polarization is
(2.6)
= χ (2) [E12 e−2iω1t + E22 e−2iω 2 t + 2E1E2 e−i(ω1 + ω 2 )t + 2E1E2*e−i(ω1 + ω 2 )t + c.c.]
+2 χ (2) [E1E1* + E2 E2* ]
The summation of nonlinear polarization can be expressed by different
components that describe processes such as SHG, SFG, different-frequency
generation (DFG) and optical rectification (OR) as shown below.
(2.7)
The geometry for SFG is shown in Figure 2.2-1. Generally, two pulsed laser
beams, including a visible beam in
and a tunable infrared beam in
16 , overlap
spatially and temporally on a sample and the reflected SFG light at the sum of the two
incident frequencies
enhanced when
is detected. (35) The intensity of SFG light is resonantly
overlaps with the resonant frequency of a molecular vibrational
mode that is both infrared and Raman active. (20) Scanning the frequency of the IR
laser therefore yields a SFG spectrum. The position and the magnitude of those
resonance peaks provide chemical and orientation information of the molecules at a
surface or an interface.
Figure 2.2-1. Schematic of the laser beams for SFG spectroscopy at an interface. (35)
The intensity of SFG light depends on the second-order susceptibility
which is a polar tensor of rank three and has 27 elements represented as
change the sign of
. We
(2)
= − χ −i(2)− j − k . For
under the inversion operation: χ ijk
centrosymmetric crystals which are invariant under the inversion operation:
(2)
χ ijk
= − χ −i(2)− j − k ,
should be zero. As a consequence, SFG is forbidden in
centrosymmetric media under electric-dipole approximation and only permitted at
surfaces and interfaces where inversion symmetry is broken.
17 When we are probing the surface or the interface, we can choose either S or P
polarization for incident beams and receive the output SFG by selecting a polarization
for the detector. (Here, P polarization refers to light whose electric field is in the
incident plane, while S polarization refers to light whose electric field is parallel to the
surface.) Different combinations of laser polarization offer corresponding Cartesian
components of
. For a surface that is azimuthally isotropic, there are only four
independent non-zero components of
:
,
,
. In the case of S-polarized visible and S-polarized infrared beams, only
and
is
non-zero and contributes to P-polarized light. In the case of S-polarized visible and Ppolarized IR beams, only
is non-zero and gives rise to the S-polarized light. In
the case of both P-polarized visible and IR lasers, all four independent elements of
contribute to the output intensity. In most of current SFG measurements, SSP,
PPP, SPS polarization combinations are chosen (e.g. SSP is given in the following
sequence: S-polarized SFG output, S-polarized visible input and P-polarized IR
input.) The relationship between the effective susceptibility with the combination of
polarizations is shown below taking Fresnel effects into account. (36)
(2)
χ eff
, ssp = L yy (ω 3 )L yy (ω 1 )Lzz (ω 2 )sin θ 2 χ yyz
(2.8)
(2)
χ eff
, sps = L yy (ω 3 )Lzz (ω 1 )L yy (ω 2 )sin θ 2 χ yzy
(2.9)
Here,
and
are the effective second-order nonlinear optical susceptibility
measured in the experiments using SSP and SPS polarization combinations.
frequency of SFG light equal to
.
18 is the
(i = x, y, or z) are the transmission
Fresnel factors, and
and
are angles between the surface normal and the input
visible beam, and the input IR beam, respectively.
Take SSP as an example,
can be fitted by Lorentzian equation as shown
below,
(2)
(2)
χ eff
, ssp = χ NR + Fsur ∑
q
Aq
(2.10)
ω 2 − ω q + iΓ q
Fsur = Lyy (ω 3 )Lyy (ω 1 )Lzz (ω 2 )sin θ 2
where
,
(2.11)
is the nonresonant background.
and
is the complex Fresnel coefficient.
are the strength, angular frequency and damping constant of the
th
resonant vibrational mode, respectively.
The intensity of SFG signal is proportional to
and the electric field of
incident beams as shown below.
I SFG =
8π 3ω 32 sec 2 θ 3
(2) 2
χ eff
IVis I IR
3
c n(ω 3 )n(ω 1 )n(ω 2 )
(2.12)
is the angel between the surface normal and SFG beam.
index of the medium at wavenumber
.
and
is the refractive
are the intensity of visible beam
and infrared beam, respectively.
As we know SFG provides chemical and orientation information of molecules,
we need describe how to relate the SFG signal with molecular characters at a surface
19 or an interface. The overall polarization of a system is the sum of the polarizations
induced on individual molecules
as shown in the equation below, (37)
(2.13)
where
is the total number of molecules in the system,
is the average
polarization of individual molecules for the entire system. The hyperpolarizability of
a molecule
and
should be mentioned here, and the relationships between
and
are shown below.
pmol = β xyz : E1E2
χ (2) =
∑
(2.14)
β xyz = N β xyz
(2.15)
al
lmolecules
is the hyperpolarizability of a molecule defined in terms of the laboratory
coordinate system. It can be converted into the hyperpolarizability defined in terms of
the molecular coordinate system through a series of rotations using Euler angles. The
relationship between the molecular axes (a, b, c) to the lav axes (x, y, z) can be seen
in Figure 2.2-2. The coordinate transformations are performed through a series of
rotation, including rotating about the z-axis by an angle
by an angle
, rotating about the x-axis
and rotating again about the z-axis by an angle
20 . In many cases, the
Figure 2.2-2. Definition of Euler angles relating lab axes to molecular axes.
symmetry of the molecule will cause many of the molecular hyperpolarizability
elements to become zero.
Take a C3V symmetric moiety of molecules (such as methyl groups) at a
surface as an example, considering a azimuthally isotropic surface, the non-vanishing
elements of
can be presented by the hyperpolarizability elements in this way:
(38) For the symmetric stretching mode of the methyl group,
(2)
χ yyz,
s =
∫
π
(2)
χ yzy,
s =
∫
π
0
0
1
N β ccc, s [cosθ (1 + r) − cos 3 θ (1 − r)] f (θ ) sin θ dθ
2
(2.16)
1
N β ccc, s [cosθ − cos 3 θ ](1 − r) f (θ ) sin θ dθ
2
(2.17)
For the asymmetric stretching mode of the methyl group, 21 ∫
(2)
χ yyz,
s =
π
0
1
− N β caa,as [cosθ − cos 3 θ ] f (θ ) sin θ dθ (2.18)
2
(2)
χ yzy,as
=
∫
π
0
1
N β caa,as cos 3 θ f (θ ) sin θ dθ
2
(2.19)
where
is the angle between the surface normal and the principal axis of the methyl
group which can be used to describe the molecular orientation.
distribution function of
is the
. The Gaussian function is commonly used for the
distribution function as the equation below. (39)
(2.20)
Here
is the mean orientation angle,
is a normalization constant, and
is the
root-mean-square width.
By combining Equation (2.18), (2.19) and (2.20), we can relate the orientation
angle
R=
with the ratio
χ yyz,as
χ yzy,as
Here,
×
Fssp
Fsps
and
=−
between
and
as shown in the equation below.
cosθ − cos 3 θ
(2.21)
cos 3 θ
are Fresnel coefficients. The ratio
can be experimentally
obtained from SFG spectra conducted using SSP and SPS polarizations. By
comparing the experimentally measured
function of
value with the calculated
value as a
, we can estimate the orientation angle of molecules at an interface.
22 2.2.2
SFG Study of Polymers
As SFG has submonolayer sensitivity and surface specificity, and provides
chemical and orientation information of the molecules at a surface or an interface, it
has been extensively applied to investigate polymer surface composition and structure
which are related to the properties and performance of polymer materials. Chen and
coworkers study the surface structure of various polymer surfaces such as poly
(methacrylate)s, poly (methyl methacrylate)/polystyrene blend, polyethylene oxide-­‐
polypropylene oxide copolymers etc. (40-42) Performing SFG experiments under
different polarization enable a quantitative analysis of molecular orientation, in
particular relating the intensities of SFG spectra to orientation information of methyl
and methylene groups. Extensive research relates the microscopic structure with
macroscopic properties such as adhesion, wettability, antifouling and antimicrobial
abilities. (43-47)
SFG was applied to monitor surface structure changes at a molecular level
under various surface treatments, such as UV irradiation and plasma treatment, wet
treatment, rubbing and sheardeposition. (48-53) Among these studies, extensive
attentions were attracted by restructuring of the surface in contact with water. (20, 5456) In the study of poly (ethylene glycol) surfaces (54), simultaneous spectral
investigations in the C–H (2800-3000 cm-1) and O–H (3000-3800 cm-1) stretch
regions showed that interactions with water induce dramatic conformational changes
of the polymer backbone.
As SFG spectroscopy is an interfacial specific technique, it widely applied to
study the buried solid-solid interfaces. (20, 55, 56) Gautam et al. used a total internal
reflection geometry to study both the sapphire–polystyrene and air–polystyrene
23 interfaces probed by SFG spectroscopy. They collected spectra related to these two
different interfaces by using two different angles of incidence of the input beams,
where only one interfacial (air–polymer or sapphire–polymer) contribution is
amplified. They found the phenyl rings of polystyrene have significantly different
orientations at these two interfaces. The molecular orientation at the buried solid–
polymer interface could also be correlated to adhesion in the case of polystyrene in
contact with the modified glass substrate. Kurian and coworkers have used SFG
spectroscopy to determine the interaction energies of various polymers in contact with
the sapphire substrate which govern the wetting, adhesion, friction, chemical reactions
and many other material and biological phenomena at interfaces. (57) Other SFG
investigations of buried interfaces including polymer–polymer and silane–polymer
interfaces were conducted by Chen and coworkers. (58, 59) In this latter case, they
succeeded in monitoring the evolution of the microscopic structure of the interface
during the diffusion of silanes.
2.2.3
SFG Study of Water
The importance of water interfaces has long been well recognized because
they’re ubiquitous and play critical roles in many natural and technological processes.
However, studies of these interfaces at the molecular level are somehow limited
because of a few available techniques. Infrared-visible sum frequency generation
spectroscopy which is interfacial specific and has submonolayer sensitivity has been
proven to be the most powerful and versatile method to obtain surface vibrational
spectra of liquid interfaces that yield information about liquid interfacial structure.
24 2.2.3.1
Vapor-Water Interfaces
The first set of vibrational spectra for the vapor/water interface was obtained
by Du and coworkers in 1993. (60) As displayed in Figure 6, the SSP spectrum of the
vapor/water interface was compared with the IR spectra of bulk ice and liquid water.
A sharp peak at 3700 cm-1 is assigned to the stretching mode of the dangling OH
bonds pointing towards the vapor side as shown in Figure 2.2-3. (61) The 3700 cm-1
peak is characteristic of the surface instead of bulk. The two broad peaks at 3400 cm-1
Figure 2.2-3. The SFG spectra of the vapor/water interface, IR spectra of bulk water
and bulk ice, and molecular structure of hexagonal ice. Upper-left: The SFG spectrum
of the vapor/water interface shows three OH bands at 3200, 3400 and 3700 cm-1.
Upper-right: side view of the bulk ice near the (0001) surface. Bottom-left: The IR
spectrum of bulk ice shows a dominant peak at 3150 cm-1. Bottom-right: The IR
spectrum of bulk water is dominated by a broad peak at ~3400 cm-1. (61)
25 and 3200 cm-1 resemble peaks for the stretching modes of the bonded OH in bulk
water and bulk ice, respectively. We call them as liquidlike and icelike peaks,
corresponding to tetrahedrally hydrogen-bonded and three-coordination hydrogenbonded water networks.
Wei and coworkers also obtained SFG spectra for the vapor/water interface with
three polarization combinations, SSP, PPP and SPS. (62) All the peaks in the PPP and
SPS spectra shown in Figure 2.2-4 are significantly weaker than those in SSP because
the OH bonds of water at the interface are tilted close to the surface normal. As a
consequence, its vibrational modes are more easily excited by S-polarized, rather than
P-polarized, IR input. In the PPP and SPS spectra, the resonant feature at 3500-3600
cm-1 can be assigned mainly to the bonded OH stretching mode of surface water
molecules with one bonded OH and one dangling OH (low-coordination water).
Infrared-visible sum frequency generation spectroscopy has been used to study
various types of water interfaces. (63-68) However, detailed interpretation of the
spectra often differs and causes a great deal of confusion. The difficulty usually arises
because of ambiguity in analyzing the spectra and because of lack of sufficiently
accurate theoretical calculation to compare with experiment. Bonn et al. reported that
the interfacial water resonance in the hydrogen-bonded region (3000-3500 cm-1)
originates from intramolecular coupling of vibrational modes, rather than from the
existence of distinct water substructures. (69) He supported his arguments based on
the spectra of uncoupled HOD peaks, which reveal a smooth transition from two
peaks to one peak in the hydrogen-bonded region as D2O is converted into HOD.
Thus, they concluded that two prominent peaks observed at water interfaces are
assigned to the symmetric stretching mode split by the Fermi resonance with the
overtone of the bending mode.
26 Figure 2.2-4. The SFG spectra of the vapor/water interface taken with (a) SSP, (b)
PPP, and (c) SPS polarization combinations. (62)
In response to Bonn’s argument, Shen and coworkers developed a phasesensitive SFG technique, allowing direct measurements of both the amplitude and the
phase of the SF response and helping in analysis and interpretation of the spectra of
water interfaces. (70, 71) In their study of the vapor/water interface, they found the
bonded-OH (or bonded-OD) spectra of pure H2O (or pure D2O) exhibit a double-peak
feature, while this feature convert into a single broad band with sufficient isotopic
dilution. The spectra are consistent with those provided by other groups. However, the
measured Im
spectra of HOD at the vapor/water interface as displayed in Figure
2.2-5 showed one positive resonance band around 3300 cm-1 and one negative
27 reaonance band around 3450 cm-1, which efficiently refuted Bonn’s argument that the
two peaks originates from intramolecular coupling of vibrational modes.
They proposed a model of the first two molecular layers of the vapor/water
interface that dominantly contribute to the SFG spectra, consisting of DAA, DDA,
and DDAA molecules in a distorted hydrogen-bonding network. Here D and A denote
single proton donor and single proton acceptor through which water molecules bond
to neighbors. As shown in Figure 2.2-5, the sharp positive peak at 3690 cm−1 is
attributed to the dangling OH stretching mode of DAA water molecules protruding at
the surface. The positive band at 3300 cm-1 and the negative band at 3450 cm-1 appear
at the same positions as the IR absorption bands of HOD in bulk ice and liquid water.
Thus, they are assigned to icelike DDAA water molecules with dipole moment
pointed towards the vapor side and liquidlike (loosely donor-bonded to molecules)
DDAA, DAA and DDA water molecules with dipole moment pointed towards the
bulk water. This work here shows that the spectra of Im
can provide a much
clearer picture of the vapor/water interfacial structure even though the overall broad
spectra is complex due to continuous variation of hydrogen-bonding geometry and
strength.
Figure 2.2-5. The Im
spectrum of HOD at the vapor/water interface probed by
phase-sensitive sum frequency generation spectroscopy, and schematic of the first two
molecular layers at the vapor/water interface occupied by DDA, DAA and DDAA
water molecules. (71)
28 2.2.3.2
Water-Hydrophilic Solid Interfaces
Hydrophilic solid surfaces are often hygroscopic. The absorbed water on solid
surfaces is critical in wetting, weathering and biological phenomenon. Miranda and
coworkers studied the structure of a water film formed on mica as a function of
relative humidity (RH) of water vapor by using SFG spectroscopy and scanning
polarization force microscopy (SPFM). (72) From the SFG spectra as RH (%)
increases shown in Figure 2.2-6, partial water coverage about 20% RH doesn’t form
an ordered water structure. At 90% RH, a full monolayer coverage is achieved,
forming an ordered ice-like structure with no dangling OD groups. This result agrees
with the prediction of molecular dynamics simulations. (73) They applied SPFM to
image the water adsorption on mica and they found that the polygonal shaped islands
that are generated by tip contact with the surface can be observed from 20% RH up to
70% RH. As RH is above 80%, the surface is nearly fully covered by an ice-like
water film, and it becomes more difficult to observe an isolated island.
Many studies were conducted for probing the water-hydrophilic solid
interfaces such as silica-water, sapphire-water and CaF2-water interfaces. The
isoelectric points of these solid surfaces determine whether the surfaces are neutral,
positive charged or negative charged, which directly influences water structure
forming on these surfaces. Take silica for an example, the surface remains neutral
when the pH of bulk water is lower than 2, becomes increasingly deprotonated as pH
increases, and is completely deprotonated and saturated with negative charges at
pH=10. (74) Water molecules can form hydrogen-bonded networks on both neutral
(SiOH) and negative charged (SiO-) surfaces. As shown in Figure 2.2-7, when the
surface is neutral, two water molecules can bind with H to O and one with O to H on
29 SiOH, while three molecules can bind with H to O of SiO- when the surface is
deprotonated. The surface field at high pH should help orient the water molecules
with hydrogen bonding to the surface and establish more ordered hydrogen-bonded
networks.
Figure 2.2-6. The SFG spectra of water (D2O) film on mica as a function of the
relative humidity (RH) at room temperature (296K) with SSP polarization, and
scanning polarization force microscopy images, showing 2-dimensional islands of
water (bright patches) produced by a brief contact of the atomic force microscope tip
near the center of the image, that induces capillary condensation around the contact
point. At relative humidity values below 70%, the clusters show a contrast of 2.5
0.5 Å. The contrast decreases down to the noise level at high humidity (80%). (72)
30 Figure 2.2-7. Possible hydrogen-bonding configuration of water molecules on
hydrophilic silica surface: (a) protonated (SiOH) surface sites, low pH; (b)
deprotonated (SiO-) surface sites, high pH; (c) structure of water/silica interface at
low pH. Red and gray spheres represent O and H atoms of water molecules; large
graygreen, pink, and white spheres represent Si, O, and H atoms of SiOH groups at
silica surface. Dotted lines indicate hydrogen bonds. (74)
This is now confirmed by SFG spectroscopy. Shen and coworkers present in
Figure 2.2-8 a set of SSP spectra of silica/water interfaces at different bulk pH. (75)
The observation of the liquidlike and icelike peaks indicates that the interfacial water
molecules form a partially ordered hydrogen-bonded network. Both peaks increase
with pH, but the icelike peak grows more significantly at high pH, indicating a betterordered water network at high pH.
Yeganeh and coworkers probed the alumina/water interface by SFG
spectroscopy and reported the isoelectric point at around pH=8. (76) From many other
researches of alumina/water interfaces, the isoelectric points range from 3 to 9.5 that
31 relate to different crystallographic plane, experimental techniques, and surface
treatments. (77-80) Adsorption of ions and charged polymers on the bare solid surface
will significantly alter the point of zero charge and influences the hydrogen-bonded
water structure as a consequence. (81)
Figure 2.2-8. The SFG spectra the water/fused silica interface as a function of pH.
Polarization combination is SSP. A spectrum of the ice/fused silica interface is shown
for comparison (filled squares). The spectra are offset vertically by 2 units for clarity.
(75)
32 To sum up, there is a coexistence of icelike and liquidlike water structures on
hydrophilic solid surfaces. Differing from the vapor/water interface, no dangling OH
(or OD) band shows up at the water/hydrophilic solid interface. The hydrogen-bonded
structures can be affected by modification of the surface through deprotonation, ion
adsorption, or molecular adsorption at the surface which determine how the interfacial
water molecules bond to the solid surface. Surface charges, and hence surface field,
can induce better ordering of interfacial water molecules perhaps even up to a few
monolayers.
2.2.3.3
Water-Hydrophobic Solid Interfaces
Water molecules should poorly wet a hydrophobic surface due to stronger
interactions among themselves compared with interactions between water and the
substrate. As air is an ideal hydrophobic surface, we would expect the vibrational
spectrum of the water/hydrophobic substrate interface to be similar to that of the
vapor/water interface. Particularly, the peak for the dangling OH at 3700 cm-1 should
show up since they won’t bond to the substrate.
Figure 2.2-9. The SFG spectra of the water/hydrophobic interfaces: (a) the
water/octadecyltrichlorosilane (OTS) covered silica interface; (b) the water/vapor
interface; (c) the water/hexane interface. (82)
33 Du et al. first presented the SFG spectra of a water/octadecyltrichlorosilane
(OTS)-covered silica interface as shown in Figure 2.2-9. (82) The dangling OH band
at 3680 cm-1 is apparent indicating that there is no hydrogen bond between water and
OTS. Instead, the weak van der Waals interaction between the free OH and the CH3
terminal group of OTS causes the red shift by 20 cm-1. Additionally, the SFG
spectrum of a water/hexane interface looks quite similar to that of the vapor/water
interface.
Hus and Dhinojwala (23) have probed the contact interface between
oil−sapphire interfaces in aqueous media using SFG spectroscopy. A transition from
an attractive contact to repulsive contact was observed above the isoelectric point
(IEP) of the sapphire substrate. Below the IEP of the sapphire substrate, the
hexadecane drops stick to the sapphire surface and Hus surprisedly observed a thin
layer of water in the adhesive contact region between hexadecane drop and the
sapphire substrate. The presence of this water layer in the adhesive contact region can
be explained due to weaker repulsive double layer and the attractive van der Waals
interactions.
34 CHAPTER III
EXPERIMENTAL
3.1
Sample Preparation
The sapphire prisms were purchased from Miller Optics with the c-axis
parallel to the prism face. The prisms were cleaned by following procedure: (1)
sonicate in toluene for more than 2 hours using a Branson model 610 sonicator; (2)
blow with nitrogen gas; (3) expose to an air plasma treatment for 2-3 minutes using a
Harrick PDC-32G scientific plasma cleaner which is connected with a Varian 3201
direct drive rotary vane pump.
Polyurethane (PU) films were obtained by spin coating 0.3% - 1% solution in
chloroform on one face of the sapphire prism (spin coating conditions: the rotating
speed is 2000 RPM and the rotating time is 1 minute), and dried in a vacuum oven at
room temperature for 12 h. Polyurethane (Mw=100 kg/mol) was purchased from
Sigma-Aldrich
and
was
used
as
received.
It
consists
of
alternating
dicyclohexylmethane-4,4’-diisocyanate and polytetramethylene oxide (PTMO) in the
chemical structure of the polymer as shown in Figure 3.1-1. Spin coating was
conducted by using a Specialty Coating Systems model P6700 spin coater.
Ultrapure distilled H2O (18 MΩ·cm) used in our study was obtained from a
Millipore filtration system. D2O was purchased from Cambridge Isotopes (D: 99.9%).
35 Figure 3.1-1. The chemical structure of polyurethane that is used in our study.
3.2
Ellipsometry Measurements
Film thickness (Polymer films were spin coated on clean silicon wafers.) was
obtained by spectroscopic ellipsometry measurements using a phase-modulated
ellipsometer purchased from MRL. The thickness of PU films was measured by a
phase-modulated ellipsometer purchased from MRL. Three angles of incident beam
,
and
were used to obtain spectra with a range from 3000 to 10000
wavenumbers. Data analysis was performed by creating a three-layer model (Si +
SiO2 + PU) and fitting the data. During fitting, the thickness of Si layer can be
assumed as 1 mm and the thickness of SiO2 layer can be obtained by measuring bare
silicon wafer using ellipsometry.
3.3
Quartz Crystal Microbalance (QCM) Measurements
Q-sense E4 operator from Biolin Scientific AB was used to study the water
uptake of polymer thin film. SiO2 coated crystal sensor X301 (5 MHz resonant
frequency) was chosen as a substrate for spin coating. The sensor crystals were spin
coated with 300 nm PU films and placed into the QCM flow cell. H2O was introduced
into the flow cell (Liquid water is contacting with polymer films.) at a rate of 0.100
ml/min-1 at 25
for certain hours until a stable baseline was obtained. D2O was
36 introduced into the flow cell and the change in frequency was recorded. The shift due
to the change in solvent occurred in <30 s. After certain minutes, H2O was again
introduced to the cell, and the frequency returned to the initial baseline. The
differences in density of H2O and D2O caused changes in the resonant frequency of
sensor crystals, and permitted calculation of the mass density of absorbed water by
polymer films. (16) The calculations can be performed based on two equations listed
below.
(3.1)
(3.2)
( C=0.177 mg.s/m2 )
Where and are the frequency differences for bare sensor crystals
and crystals coated with polymer films (n represents nth overtone) caused by solvent
exchange. Using Equation 2 and Equation 3, we can know frequency change cause by
the absorbed water 3.4
and water uptake value . Sum Frequency Generation Spectroscopy Apparatus and Measurement
The SFG measurement involves a visible pulse at 798 nm overlapping
37 spatially and temporally with a tunable infrared pulse at 3-10 µ m . The two pulses
have a 1 ps width and 1 kHz repetition rate. As shown in Figure 3.4-1, the SFG
apparatus has two laser sources, a Ti: Sapphire laser and a Nd: YLF laser. Millenia
and Tsunami combine together to generate the first laser. Millenia produces a solidstate green laser whose output (at 532 nm with the power of 5W) is directed into
Tsunami. In Tsunami, a Ti: Sapphire laser (mode locked) at 800 nm wavelength and
1 Watt power with an 82 MHz repetition rate and a pulse width of 150 femtosecond
is generated and used to drive the amplifier. The laser is stretched to 1 picosencond
and it forms the seed before entering into the amplifier. The second laser produced by
Empower is a pulsed, green, 527 nm beam with 1 kHz repetition rate and 30 Watt
power. It is also pumped into the amplifier (Spitfire). In Spitfire, the seed and the
pump pass through a Ti: Sapphire laser rod and the input pulse from Tsunami is
amplified more than 6 orders of magnitude in energy. The amplified pulse is then
compressed and directed into optical parametric amplifier (OPA) with the width of ~1
ps and ~ 1 Watt power.
In OPA, the input beam is split into two limbs, including the majority of
energy (~ 96%) is reflected and used for pumping the OPA and the rest (~ 4%) is
transmitted and used to produce a white light continuum which provides the seed. The
major portion of the amplified beam is split into two pump beams, the first path
(15%) and the second path (85%). The seed pulse and the first pumping path overlap
in a beta barium borate (BBO) crystal and signal and idler are generated. The
amplified idler beam passes through the crystal and is reflected back by a mirror
WLR3 (diffraction grating), and then overlaps with the second pumping path in BBO
crystal for final amplification. These beams are sent to an AgGaS2 crystal (different
frequency mixing (DFM)) to generate an IR pulse and a visible beam output. The IR
38 beam wavelength can be tuned by rotating the BBO crystal which tunes the
wavelengths of the signal and idler beams. By rotating the diffraction grating and
DFM crystal, we can maximize the output intensity.
The IR and visible output beams from OPA are adjusted to overlap spatially
and temporally on the sample surface, and the produced SFG beam is directed into a
photomultiplier tube (PMT) and the SFG signal is detected by a Stanford Research
Systems SR400 gated photon counter. Simultaneously, a Stanford Research Systems
SR850 DSP lock-in amplifier detects IR intensity. Then the SFG intensity (number of
photon counts) is normalized with the IR intensity and plotted as a function of the IR
wavenumber as shown in a SFG spectrum.
Figure 3.4-1. The schematic diagram of SFG apparatus.
39 3.5
Total Internal Reflection (TIR) Geometry Combined with SFG Measurement
We combined TIR with SFG measurement and chose sapphire prisms as a
model substrate in our study because SFG signal can be enhanced by 1-2 order of
magnitude near critical angles in this geometry (The critical angles were calculated
using Snell’s law). As shown in Figure 3.5-1, a sapphire prism is coated with the
polymer film in contact with water or air (or other medium). The refractive index of
water, air, polyurethane and sapphire used in the calculation are 1.33, 1, 1.51 and
1.76, respectively. By performing experiments at proper incident angles, we can
control angles between refracted beams and the prism face that is coated with
polymer. Total internal reflection takes place when those angles are near critical
angles, which allow us to selectively probe the sapphire-polymer interface and the
polymer-air (or water) interface. In our study, we used incident angles (with respect
Figure 3.5-1. The schematic of combining TIR geometry with SFG measurements.
40 to the surface normal of the sapphire prism face) of 42, 16, 2 and 2 degrees to probe
PU-air, PU-H2O (and PU-D2O), PU-sapphire and PU-vapor interfaces, respectively.
3.6
Sample Cell and the Humidity Controlling Set-Up
We designed a SFG cell that can be used to add water or water vapor in
contact with the polymer layer as Figure 3.6-1 shown. Water between polymer and
the sample cell was sealed well to prevent any loss of water during experiments. The
stainless steel cell was cleaned by the same procedure of cleaning sapphire prisms
before using.
Figure 3.6-1. Diagram of the sample geometry for SFG measurements. Liquid water
and water vapor can be sealed well in the gap between the sapphire prism and the
stainless steel sample cell.
A humidity-controlling set-up was also built to control the real-time RH and
this setup was integrated with the SFG setup to continuously flow water vapor on the
top of polymer layer. Figure 3.6-2 shows the humidity meter (iTHX-D3 from Omega)
to monitor humidity values during the experiments. The controlled humidity was
obtained by adjusting the flow rate of N2 gas bubbling through water and the flow rate
41 of dry N2 gas mixing with water vapor to lower the humidity. The sample was fixed to
the cell exposing to water vapor with certain RH and equilibrated for 15 minutes
before we collected the SFG spectra.
Figure 3.6-2. Diagram of the humidity-controlling set-up. The water container is
made of glass and other parts are made by stainless steel.
42 CHAPTER IV
RESULTS AND DISCUSSION
4.1
Calculating Contributions of SFG Signal from Two Interfaces as a Function of
the Film Thickness
Before we test whether water molecules can penetrate polymer films and reach
the polymer-sapphire interface, we need to understand the contributions of SFG signal
from two interfaces that are involved in our measurements. For dry samples, we have
polyurethane-air and polyurethane-sapphire interfaces that can contribute to the SFG
signal; while we have polyurethane-water and polyurethane-sapphire interfaces for
samples exposed to liquid water. A mathematical model for the three-layered system
developed by Guifeng et al. (83) was used to demonstrate the dependence of
interference effects of signals from two interfaces on the PU film thickness as a
function of incident angles. We have focused our analysis for incident angles of
and
,
because these were the conditions we used to probe PU-sapphire, PU-
water and PU-air (or PU-vapor) interfaces, respectively.
Figure 4.1-1 presents the ratio of the signals from two interfaces as a function
of film thickness (smaller ratio indicates weaker interference effects). As shown in
Figure 4.1-1, the interference of the upper interface to the signal of the bottom
interface decreases as the thickness of the PU film increases. The interference of the
bottom interface to the signal of the upper interface fluctuates as varying the thickness
43 of the PU film. Ratios for all the four curves are smaller than 0.1 when the film
thickness ranges from 260 nm to 300 nm. We can conclude that when we use PU
films with thickness locate in this range, interferences of signals between two
interfaces will be reasonably minimized.
Figure 4.1-1. Predictions of interferences of the SFG signal for two interfaces as a
function of the thickness of polymer films by a three-layered structural model. Red
dashed line is the ratio of signals from PU-air interface over PU-sapphire interface at
incident angle 2 degrees. Blue dashed line is the ratio of signals from PU-water
interface over PU-sapphire interface at incident angle 2 degrees. Pink solid line is the
ratio of signals from PU-sapphire interface over PU-air interface at incident angle 42
degrees. Black solid line is the ratio of signals from PU-sapphire interface over PU-air
interface at incident angle 16 degrees.
In this model, we assume that the refractive index of all materials in our study
is a real quantity, and absorption in the IR wavelength is neglected. When studying
the possible four-layered system (A water film exists between the sapphire and the PU
film, and the thick water layer sits on the top of the PU film.), we assume that the
three-layered model can work well because the water film at the bottom interface is
44 too thin to influence the optical results.
4.2
The Thickness of Polymer Films by Ellipsometry
Figure 4.2-1 shows the thickness of PU films (They are spin coated on silicon
wafers) measured by ellipsometer as a function of PU solution concentration. The
thickness increases as increasing the solution concentration. A 292 ± 4 nm thick PU
films were obtained using 1 wt. % solution and this film thickness was used for the
SFG measurements.
Figure 4.2-1. Diagram of the thickness of PU films as a function of the solution
concentrations varying from 0.3% to 1%. Each data point is the average result of three
different samples.
4.3
Water Uptake of Polymer Films by QCM
As shown in the Figure 4.3-1, we can clearly see the change in frequency
because of the solvent exchange (H2O  D2O  H2O). The eight lines correspond to
45 bare and coated sensors under 3rd, 5th, 7th and 9th overtones. We collect the frequency
change for bare sensors
(n represents nth overtone) and sensors coated with
PU films
, and calculate the contribution of water
and water
uptake value
. The data for different sensors are shown in Table 4.3-1.
According to
, we can calculate that the weight percentage of water in the PU
film and between the sensor and the PU film is 0.3% - 1.7% (wt%).
Figure 4.3-1. The QCM spectra of the bare sensor and the sensor spin coated with the
polyurethane film (300nm by ellipsometry) at 3rd, 5th, 7th and 9th overtones.
We also use microbalance to measure the water uptake of PU. We soak five
PU beads in H2O for 5 days, and then take the beads out of H2O and dry the beads
surface. We measure the weight of PU beads before and after soaking in H2O, which
are 54.8 mg and 55.7 mg. The water uptake of PU is 1.6%. This method can only give
approximate water uptake value of PU. By comparing the results of these two
46 techniques, we can confirm that PU absorbs 1-2 wt.% water. As we’ve already known
that the polymer films can absorb water, the interfacial specific technique SFG will be
used to detect whether water reaches the polymer-substrate interface.
TABLE 4.3-1. QCM experimental data and calculated data
-1
(s )
(s-1)
(mg/m2)
4.4
Sensor 1
Sensor 2
Sensor 3
Sensor 4
n=3
-61.41
-64.59
-59.49
-63.24
n=5
-49.9
-44.75
-50.85
-51.3
n=7
-43.47
-43.26
-43.19
-42.7
n=9
-38.07
-36.81
-37.62
-37.71
n=3
-63.72
-66.06
-
-63.27
n=5
-53
-45.55
-
-52
n=7
-45.08
-44.66
-
-43.44
n=9
-39.87
-37.44
-
-38.25
n=3
3.81
2.433
-
0.0495
n=5
5.15
1.325
-
1.16
n=7
2.66
2.317
-
1.043
n=9
2.979
1.044
-
0.8937
PU-Air Interface and PU-Sapphire Interface of Samples in the Ambient
Condition by SFG
Figure 4.4-1 shows the SFG spectra in PPP polarization for the PU-air
interface (a) and the PU-sapphire interface (b) respectively. The fitting parameters are
shown in Table 4.4-1. From the PU-air interface, we see three characteristic peaks at
2800, 2850, and 2920 cm-1. The three peaks shown in IR spectra provided in appendix
47 and can be assigned to α-CH2 (s) stretching, normal CH2 (s) stretching and normal
CH2 (as) stretching modes, respectively. (51, 84-86) The spectrum of the PU-sapphire
interface reveals three peaks at 2850, 2940 and 3650 cm-1. Assignment of the band at
2940 cm-1 is less definite. Even though it was attributed to the Fermi resonance of the
Figure 4.4-1. The SFG spectra in PPP polarization for the PU-air interface (a) and the
PU-sapphire interface (b). The solid lines are fit to a Lorentzian equation.
48 CH2 (s) stretch with the overtone of the methylene deformation in literature, this
assignment is not reasonable in our study because we didn’t observe comparable
intensity of CH2 (s) stretch at the PU-sapphire interface. Thus, the 2940 cm-1 peak is
tentatively assigned to an asymmetric CH2 vibration of soft segment (PTMO) moiety.
(54, 87) The 2940 cm-1 peak is significantly different from those for the PU-air
interface, indicating the interaction between PU and sapphire. We attributed the 3650
cm-1 peak to the surface OH of sapphire in contact with PU. We can see, based on the
magnitude of the acid-base interactions, the surface OH peak shifts to lower
wavenumber (The 3720 cm-1 peak is assigned to free surface OH peak on the sapphire
surface.), which confirm the interaction between PU and sapphire. (75)
TABLE 4.4-1. SFG fitting parameters with PPP polarization deduced from Figure
4.4-1 for the experiments of probing the PU-air interface and the PU-sapphire
interface.
PU-air interface
PU-sapphire interface
2800
11
9
2850
12
32
2860
15
84
2940
23
194
2920
19
86
3650
105
1070
TABLE 4.4-2. SFG fitting parameters with SSP polarization deduced from Figure
4.4-2 for the experiments of probing the PU-air interface and the PU-sapphire
interface.
PU-air interface
PU-sapphire interface
2800
19
35
2850
0
0
2850
14
75
2950
22
59
2910
19
58
3650
103
617
49 We also took spectra of the PU-air and the PU-sapphire interfaces by SSP
polarization as show in Figure 4.4-2. Similar peaks were observed in spectra of both
PPP and SSP polarizations. The fitting results are summarized in Table 4.4-2.
Figure 4.4-2. The SFG spectra in SSP polarization for the PU-air interface (a) and the
PU-sapphire interface (b). The solid lines are fit to a Lorentzian equation.
50 4.5
PU-Water Interface and PU-Sapphire Interface after Exposure of Samples to
Liquid Water by SFG
Figure 4.5-1 (a) shows the SFG spectrum of PU-H2O interface using PPP
polarization. The fitting parameters are summarized in Table 4.5-1. We observed two
H2O peaks at 3100 and 3350 cm-1 which are assigned to strongly tetrahedrally
coordinated (ice-like) and lower coordination (liquid-like) hydrogen-bond stretch,
respectively. (57) The peak at 2940 cm-1 comes from PU-sapphire that is different
from hydrocarbon peaks at PU-air interface, indicating the reorientation of polymer
chains because of the interaction between PU and H2O. The Figure 4.5-1 (b) shows
the SFG spectrum of PU-D2O interface. The observation of peaks at 2410, 2500 and
2940 cm-1 are consistent with those in Figure 4.5-1 (a), corresponding to ice-like,
liquid-like D2O bands and CH2 (as) stretch of PTMO moiety. The ice-like water
bands in Figure 4.5-1 indicate an ordered layer of water is formed on PU surface. As
suggested by other SFG studies of water/hydrophilic solid interfaces (such as silica
and polyethylene oxide (PEO)), (62, 88, 89) the hydrogen bonds between water and
silanol groups (SiOH) (or PEO headgroup) contribute to the ordering of water. We
TABLE 4.5-1. SFG fitting parameters with PPP polarization deduced from Figure
4.5-1 for the experiments of probing the PU-H2O interface and the PU-D2O interface.
PU-H2O interface
PU-D2O interface
2950
17
57
2410
38
140
3100
127
1107
2500
89
918
3350
80
239
2940
25
423
51 can conclude that there are hydrogen bonds forming between polar groups (including
ether, ester and amine groups) of PU chain and water, which causes the coexistence of
ice-like and liquid-like water structures at the PU-water interfaces.
Figure 4.5-1. The SFG spectra in PPP polarization for the PU-water interfaces: (a) the
PU-H2O interface and (b) the PU-D2O interface. The solid lines are fit to a Lorentzian
equation.
52 The Figure 4.5-2 (a) shows the spectrum of the PU-sapphire interface after
liquid H2O was filled between the cell and the sample using PPP polarization. In this
SFG Intensity (a. u.)
figure, we observed two H2O bands at ~3150 and ~3400 cm-1, and a PU peak at 2940
(a)
300
200
100
0
2800
3000
3200
3400
-1
Wavenumber (cm )
3600
3800
Figure 4.5-2. The SFG spectra in PPP polarization for the PU-sapphire interface upon
exposure to liquid water. Both H2O (a) and D2O (b) reaches the PU-sapphire
interface. The solid line in (a) is a guide to the eye. The solid line in (b) is fit to a
Lorentzian equation.
53 cm-1, which indicates the ingress of water to the PU-sapphire interface. In addition,
we observed a peak at ~3700 cm-1 that is associated with water and surface OH
groups. The fitting paremeters are provided in Table 4.5-2. The interpretation and
assignment of this peak is less definite because of overlapping of vibrational
frequencies for water and surface OH. Our system is also very complicated, including
water in contact with PU, water in contact with sapphire and possible sapphire in
contact with PU. Thus, the opposite directions of dipole moment would make our
interpretation more difficult.
TABLE 4.5-2. SFG fitting parameters with PPP polarization deduced from Figure
4.5-2 for the liquid H2O and D2O experiments of probing the PU-sapphire interface.
PU-sapphire interface
PU-sapphire interface
2940
23
-442
2400
46
240
3140
102
1401
2490
55
398
3370
119
868
2950
27
640
3690
28
113
3620
93
1574
We replaced H2O with D2O and measured the SFG spectra of the PU-sapphire
interface in the presence of D2O, which makes our interpretation easier because the
D2O peaks are between 2300—2800 cm-1. Figure 4.5-2 (b) shows the spectrum of
PU/D2O/sapphire interface. The peaks at 2400 and 2490 cm-1 are assigned to ice-like
and liquid-like D2O peaks, which indicate the presence of a water layer confined
between PU and sapphire. The peak at 3620 cm-1 is assigned to sapphire OH peak in
contact with D2O. A previous study of the hexadecane/D2O/sapphire interface by Ping
54 et al. suggested this peak corresponded to the surface OH band. (23) We also
observed this peak at the D2O-sapphire interface as shown in Figure 4.5-3, which
confirms the assignment. The sapphire OH peak in contact with polyurethane is
expected to be near 3650 cm-1. The red shift to 3620 cm-1 indicates that the sapphire
OH is now in direct contact with D2O, and as the water layer confined between
polyurethane film and sapphire substrate disrupts their bonds as a consequence.
Figure 4.5-3. The SFG spectrum in PPP polarization for the D2O/sapphire interface.
The solid line is used to guide eyes.
4.6
Sapphire-Water Vapor (or Dry N2 Gas) Interfaces by SFG
In our water vapor experiments, the humidity-controlling set-up was integrated
with SFG set-up. Even though we used a careful cleaning procedure to clean our
samples, potential contamination that came from the humidity-controlling set-up was
a big issue as SFG is very sensitive to organic contaminations. Any organic
contamination that absorbed on our samples can be detected by SFG spectroscopy and
55 can cause additional peaks in the SFG spectrum. Thus, it will bring us confusion and
difficulty for interpreting the real SFG spectrum. At the beginning of building the
humidity-controlling set-up, all the tubes we used were polymeric tubes such
polyethylene (PE), polyvinyl chloride (PVC) and silicone rubber. Because of
evaporative additives were added in these tubes, they produced serious
contaminations during experiments. Thus, we replaced all the polymeric tubes with
stainless steel tubes and checked again whether the contamination issue is solved.
To make sure the whole experimental set up is clean enough, we collected
spectra of the sapphire-water vapor interface before we studied the PU-sapphire
interface after the humidity-controlling set-up was integrated to the sample cell as
show in Figure 4.6-1. The cleaned blank sapphire prism was first exposed to dry N2
gas (0% RH), then exposed to water vapor with 72% RH, and finally exposed to dry
N2 gas again for 30 minutes and 60 minutes. We didn’t see hydrocarbon peaks in the
region 2700-3000 cm-1 after N2 gas passed through the humidity chamber and blew
the sapphire surface, indicating that there is no organic contamination caused by the
humidity-controlling set-up. The sharp peak at 3730 cm-1 in the red curve is assigned
to the free surface OH of sapphire which confirms that there is no absorbate on
sapphire. This sapphire OH peak became broader and red shifted to lower
wavenumber after the sample is exposed to water vapor because water molecules
absorbed to sapphire surface and interacted with surface OH groups. After the sample
was exposed to dry N2 gas again, the 3730 cm-1 peak partially recovered, which
means a non-full water coverage on sapphire surface. The peak intensity increased
with increasing the period of time for exposing the sample in the dry N2 because of
56 Figure 4.6-1. The SFG spectra of the Sapphire-water vapor (or dry N2 gas) interface
in PPP polarization after the humidity-controlling set-up was integrated to the sample
cell. The solid lines are used to guide eyes.
less water coverage. The recovery of free OH peak also indicates that sapphire surface
is covered by water instead of organic moieties. In the whole experiment, we didn’t
found organic contamination issue, and thus the humidity-controlling set-up is clean
enough and can be applied for our water vapor experiments.
4.7
PU-Sapphire Interface after Exposure of Samples to Water Vapor by SFG
For comparing with the spectra of the PU/water/sapphire interfaces, we also
collected the SFG spectra of the PU-sapphire interface after samples are exposed to
water vapor. Figure 4.7-1 shows the spectra of the PU-sapphire interface that were
collected when we exposed the sample to H2O vapor and gradually increased the RH.
57 Figure 4.7-1. The SFG spectra of the PU-sapphire interface that are collected at
various RH of H2O vapor in PPP polarization. These spectra are collected at 0%,
23%, 48% and 82% RH, respectively. The solid lines are fitted to a Lorentzian
equation.
The fitting parameters are shown in Table 4.7-1. The intensity of the 2940 cm-1 peak
assigned for PU peak increases as RH increases, while the intensity of the sapphire
OH peak at 3650 cm-1 deceases when RH reaches 82%. All these changes indicate
H2O molecules reach the buried interface and interact with PU and the sapphire
substrate. The intensity of the region from 3100 cm-1 to 3400 cm-1 increases slightly
as we increased RH. Different from the results of liquid H2O experiments, we did not
observe any obvious features for ice-like and liquid-like water in the humidity
experiments, which indicates that water molecules at the interface don’t form a
58 uniform monolayer to disrupt PU-sapphire bonds. This interpretation is also supported
by the existence of 3650 cm-1 peak in the humidity experiments in which water at the
interface partially disrupt the interaction between polymer and substrate. We observed
a peak at ~3550 cm-1. For interpreting this peak correctly, we did humidity
experiments by using D2O vapor.
TABLE 4.7-1. SFG fitting parameters with PPP polarization deduced from Figure
4.7-1 for the water vapor experiment of probing the PU-sapphire interface at various
RH (%) of H2O vapor.
0%
23%
48%
82%
230
284
349
390
25
22
20
21
311
420
372
233
52
56
53
46
2591
2368
2504
2293
86
80
83
86
Figure 4.7-2 shows the SFG spectra from D2O vapor experiment. Table 4.7-2
provides the fitting parameters and Table 4.7-3 summarizes the peak assignments for
H2O, D2O and PU next to various interfaces in the thesis. The observation of intensity
changes of PU hydrocarbon peak and sapphire OH peak are consistent with those
shown in H2O vapor experiments. In addition, we also observed signal of D2O at
~2630 cm-1 and ~2700 cm-1 which are at higher wavenumbers than those for icelike
and liquidlike D2O peaks. This is again consistent with the results for the H2O vapor
that the water molecules do not form a highly coordinated hydrogen-bonded water
layer.
59 Figure 4.7-2. The SFG spectra of the PU-sapphire interface that were collected at
various RH of D2O vapor. These spectra are collected at 0%, 23%, 53% and 80% RH,
respectively. The solid lines are fitted by Lorentzian equation.
Wei al et. obtained SFG spectra for the vapor/water interface and suggested
that the resonant feature at 3500-3600 cm-1 could be assigned mainly to the bonded
OH stretching mode of water molecules with one bonded OH and one dangling OH
(low-coordination water). (62) Another study of sorption of water into a poly (2methoxyethyl acrylate) film by Morita al et. pointed out that two bands at 3628 and
3558 cm-1 can be assigned to the antisymmetric and symmetric OH stretching modes
of water with the C = O ⋅ ⋅ ⋅ H − O type of hydrogen bond. (90) The three peaks
observed at ~3550, ~2630 and ~2700 cm-1 in our spectra may be assigned to lowcoordination water or water with the C = O ⋅ ⋅ ⋅ H (D) − O type of hydrogen bond.
60 TABLE 4.7-2. SFG fitting parameters with PPP polarization deduced from Figure
4.7-2 for the water vapor experiment of probing the PU-sapphire interface at various
RH (%) of D2O vapor.
0%
23%
53%
80%
350
350
386
394
32
24
22
23
2294
1743
1853
1801
88
87
93
90
0
648
825
870
0
83
95
101
0
92
19
18
0
30
14
12
TABLE 4.7-3. Relevant peak assignments for H2O, D2O and PU next to various
interfaces.
Origin
Peak position (cm-1)
Surface OH in contact with PU
3650
Surface OH in contact with D2O
3620
Liquid-like H2O network
~3400
Ice-like H2O network
~3150
PTMO segments CH2 symmetric
2940
CH2 symmetric
2850
CH2 asymmetric
2920
PU CH2-O symmetric
2795
Liquid-like D2O network
~2500
Ice-like D2O network
~2400
Low-coordination H2O
3550
Low-coordination D2O
2630 and 2700
61 4.8
The Effect of Hydrophobicity for Polymer Coating on Water Transport
The liquid repellency (lyophobicity) is of very importance for organic
coatings. Extensive studies were conducted for controlling wetting properties of a
surface, which is important in specialty coatings like corrosion resistant, anti-fog, and
anti-ice coatings, and in engineering devices for a variety of technological
applications such as micro-fluidics, self-cleaning surfaces, bio-mimetic surfaces. (9195) Surface wettability control can be achieved by changing surface chemistry and
surface roughness. (96) Fluorinated surfaces are widely used to enhance water
repellency because of low surface energy. However, bulk fluorinated materials are
generally expensive and difficult to process. (97) Instead, surface treatments such as
plasma chemical vapor deposition (PCVD) have been applied to coat fluorinated layer
on the surface. In our lab, PCVD technique was well built and used to create
superhydrophobic surfaces. Taking this advantage, we coat a fluorinated layer on the
top of PU film by PCVD and detect whether it can prevent water transport through the
organic coating. Contact angles (CA) are used to prove the success of PCVD and SFG
is used to measure the existence of water at the polymer-sapphire interface.
Figure 4.8-1. The contact angle images of the water droplet on (a) the polyurethanecoated sample and (b) the fluorinated layer-coated sample.
62 As shown in Figure 4.8-1, the contact angle for the polyurethane film spin
coated on the silicon wafer is 86 (obtain the same angle when testing different places
on two samples). Then PU-coated samples were coated with a fluorinated layer on the
top by PCVD technique. The contact angle of fluorinated samples increases to 110
(also obtain the same angle by testing different places on samples). It indicates that
the fluorinated layer was successfully deposited on PU films. The controlling sample
(the blank silicon wafer coated with a fluorinated layer) also has similar contact angle
with the three-layer sample.
Figure 4.8-2. The SFG spectra of the PU-sapphire interface (of plasma treated
samples) that were collected at various RH of D2O vapor. These spectra are collected
at 0%, 23%, 49% and 69% RH, respectively. The solid lines are guides to the eyes.
63 The ellipsometry was applied to measure the thickness of the fluorinated layer.
We obtained the thickness for this layer by measuring the thickness of polymer film
before and after plasma treatment and subtracting the former thickness from the latter
one. The result was 30-40 nm.
As we have developed an experimental protocol to study water at the
polymer-sapphire interface using sum frequency generation (SFG) spectroscopy, we
also collected the SFG spectra of the PU-sapphire interface after samples (plasma
treated) were exposed to D2O vapor. As shown in Figure 4.8-2, the intensity of the
peak at 2940 cm-1 assigned for PU peak increases as RH increases, while the intensity
of the sapphire OH peak at 3650 cm-1 deceases when RH is high. We also observed
signal of D2O at 2600-2700 cm-1 region that are at higher wavenumbers than those for
icelike and liquidlike D2O peaks. All these changes indicate the hydrophobic
fluorinated layer can’t prevent the ingress of D2O molecules to the buried interface.
Water molecules do not form a highly coordinated hydrogen-bonded water layer at
the PU-sapphire interface.
64 CHAPTER V
CONCLUSION
In conclusion, we have studied water at polymer-solid interfaces using
surface-sensitive sum frequency generation spectroscopy. We have used this setup to
study penetration of water through a model polyurethane film after exposure to liquid
water and water vapor. In the case of liquid water experiments, water reaches the PUsapphire interface, forming a highly coordinated hydrogen-bonded layer and
disrupting the bond between polyurethane and sapphire. The water layer is ultra thin
because of the observation of the hydrocarbon peak from PU. In the case of water
vapor, we observed water molecules penetrating to the PU-sapphire interface.
However, water molecules are unable to form a fully covered layer and they partially
Figure 5-1. A model illustrating the PU-sapphire interface in the presence of water
molecules under liquid water and water vapor conditions, respectively.
65 disrupt the PU-sapphire bonds. A simple model illustrating the results is shown in
Figure 5-1. The direct observations of water at the buried interface in liquid water and
humidity experiments have important impacts on adhesion of polymer coatings to
solid surfaces, and also provide useful information for designing coating and
preventing corrosion.
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76 APPENDIX
The IR Spectrum of Polyurethane:
Figure A1 shows the IR spectrum of PU obtained in the ambient condition.
Three peaks at 2795, 2850 and 2920 cm-1 were observed from the spectrum which
also presented in the SFG spectrum of the PU-air interface.
Figure A1. The IR spectrum of PU in the ambient condition.
77